1. Field of the Invention
This invention relates to improvements in a center core installed to the center portion of a disc-shaped recording medium and used for the purpose of making a so-called magnetic chucking for the disc-shaped recording medium to a turn table of a disc drive device, and additionally to a production process for the center core.
2. Description of the Prior Art
Hitherto a variety of center cores for disc-shaped recording mediums have been proposed and put into practical use. Of these center cores, one shown in FIG. 14 has been known, in which the center core 101 includes a flange section 102 for supporting a disc-shaped recording medium 201. A generally cylindrical section 103 having a bottom wall 103 is formed radially inward of the flange section 102. The cylindrical section 103 is formed at the central portion of the bottom wall with a supportable surface 104 through which the center core 101 is mounted on a turn table 401 of a disc drive device which will be shown in FIGS. 22 to 27. A projecting section 105 is formed radially outward of the supportable surface 104. A spindle shaft insertion hole 106 is formed at the central portion of the above-mentioned supportable surface 104. A drive pin engaging hole 107 is formed radially outward of the spindle shaft insertion hole 106 and located to extend throughout the supportable surface 104 and the projecting section 105 for adjusting the magnetic attraction force.
This conventional center core 101 is produced as follows:
At a first step shown in FIGS. 15A and 15B, a pilot through-hole 302 is formed at its central portion of a sheet 301 of a magnetic metal such as stainless steel or the like. Additionally, a plurality of arcuate cutouts 304 are formed along a circle (not shown) whose center corresponds to the center of the pilot through-hole 302. Accordingly, the arcuate cutouts 304 are circularly arranged in which a non-cutout or flat wall portion 303 is formed between the adjacent arcuate cutouts 304.
At a second step shown in FIGS. 16A and 16B, the flange section 102 and the cylindrical section 103 having the bottom wall are formed by making a press working on a radially inward side relative to the circularly arranged cutouts 304. The cylindrical section 103 formed projecting toward a lower surface side of the center core 201.
At a third step shown in FIGS. 17A and 17B, burring is applied to the pilot through-hole 302 thereby forming the spindle shaft insertion hole 106.
At a fourth step shown in FIGS. 18A and 18B, a drive pin engaging hole 107 is formed to be located radially between the spindle shaft insertion hole 106 and a peripheral cylindrical wall of the cylindrical section 103.
At a fifth step shown in FIGS. 19A and 19B, a press working is made on the bottom wall of the cylindrical section 103 in a direction from a lower surface side of the metal sheet 301 so as to cause a central portion of the cylindrical section 103 to project toward the upper surface side of the metal sheet 301. Accordingly, a shallow dish-shaped generally cylindrical section 104 having a top wall is formed at the central portion of the cylindrical section 103. Simultaneously, an annular projecting section 105 is formed along the outer periphery of the shallow dish-shaped cylindrical section 104 in a manner to surround the shallow dish-shaped cylindrical section 104. The projecting section 105 functions to adjust a magnetic attraction force to be applied to the center core 101.
At a sixth step shown in FIGS. 20A and 20B, the circularly arranged arcuate non-cutout portions 303 are cut out thereby separate the center core 101 from the metal sheet 301. The above-mentioned top wall of the cylindrical section 104 serves as a supportable surface 104a through which the center core 101 is mounted on the turn table.
As shown in FIGS. 21A and 21B, the cylindrical section 103 of the center core 101 is inserted into a core installation hole 201a formed at the central portion of a disc-shaped recording medium 201 in a direction from the upper surface side of the disc-shaped recording medium 201, so that the flange section 102 is placed on the upper surface of the disc-shaped recording medium 201. At this time, the flange section 102 of the center core 101 is bonded to the upper surface of the disc-shaped recording medium with an adhesive 202 thereby maintaining the central portion of the disc-shaped recording medium 201. In other words, the center core 101 is installed to the central portion of the disc-shaped recording medium 201.
As shown in FIGS. 22 and 23, the projecting section 105 for adjusting the magnetic attraction force is attracted by a magnet 402 installed to the turn table 401 of a disc driving device, in which the supportable surface 104a is positioned on a core support surface 403. At this time, a spindle shaft 404 located at the central portion of the core support surface 403 is inserted into the spindle shaft insertion hole 106, while a drive pin 405 disposed radially outward of the core support surface 403 is brought into engagement with the drive pin engagement hole 107. As a result, rotation of the turn table 401 is transmitted to the center core 101. FIG. 22 illustrates a case where the diameter of the core support surface 403 of the turn table 401 of the disc drive is formed relatively small or the minimum, while FIG. 23 illustrates a case where the diameter of the core support surface 403 is formed relatively large or the maximum.
However, the following drawbacks have been encountered in the above conventional center core 101:
(a) The supportable surface 104a is formed annular and around the spindle shaft insertion hole 106. The supportable surface 104a is adapted to be in direct contact with the core support surface 403, and therefore it is required that a whole area of the supportable surface 104a which area contacts with the core support surface 403 have a high surface precision. Accordingly, in this case, it is difficult to have the high surface precision throughout the whole area as compared with a case where only a part is required to have such a high surface precision. PA1 (b) Although it is ideal that the supportable surface 104a is formed as a flat horizontal plane, the supportable surface 104a is in fact inclined to form an inclined generally frustoconical surface which rises in a direction of from the outer peripheral portion toward the central spindle shaft insertion hole 106 with a gentle slope, as shown in FIGS. 24 and 25. Such inclination of the supportable surface 104a is unavoidably formed under the phenomenon of springback of the metal sheet when a press working is made on the metal sheet during production of the center core 101. When the supportable surface 104a of the above inclined shape is placed on the core support surface 403 of the minimum diameter as shown in FIG. 24, the projecting section 105 for adjusting magnetic attraction force, located radially outward of the supportable surface 104a is brought into contact with the upper surface of the magnet 402 of the turn table 401. As a result, the center core 101 will float or separate upward from the core support surface 403 of the turn table 401. Otherwise, in a case where the center core 101 is placed on the core support surface 304 having the maximum diameter as shown in FIG. 25, the projecting section 105 for adjusting magnetic attraction force may be prevented from contacting with the upper surface of the magnet 402; however, a clearance .delta. between the projecting section 105 and the magnet 402 becomes smaller so that the magnetic attraction force obtained by the magnet 402 will change.
Besides, as shown in FIGS. 26 and 27, the disc-shaped recording medium 201 is unavoidably lowered to a level lower than a standard level indicated by a dot-dash line. As a result, a contacting force of an upper magnetic head 411 against the disc-shaped recording medium 201 becomes too weak, while a contacting force of a lower magnetic head 412 becomes too strong. Therefore, it is difficult to obtain a stable output of the disc-shaped recording medium 201. In FIGS. 26 and 27, the reference numeral 211 denotes a shell in which the disc-shaped recording medium 201 is rotatably stored.